1. Field of the Invention
The invention relates to the controlled plasma-enhanced chemical vapor deposition and surface modification of silicon onto a substrate at low substrate temperatures using a hydrogen and silane gas plasma, and according to a preferred embodiment, the selective deposition and modification of silicon onto predetermined first surface areas of a substrate while preventing silicon accumulation on unintended second surface areas of the substrate in the course of a process in which silicon is deposited and accumulated in predetermined intended areas of the substrate, such as during the production of semiconductor devices.
2. Description of Prior Art
Plasma-Enhanced Chemical Vapor Deposition (PECVD) processes are well known for the deposition of silicon, including amorphous, microcrystalline, polycrystalline and/or epitaxial silicon, at low substrate temperatures (less than 300.degree. C.) in a variety of electronic semiconductor device structures. Also, atmospheric pressure and low pressure chemical vapor deposition and ultra high vacuum chemical vapor deposition are used at higher temperatures for polysilicon and epitaxial silicon deposition.
It is known to selectively deposit silicon on certain areas of a substrate by epitaxial growth thereon at temperatures greater than 600.degree. C., using chlorinated silanes and ultrapure gases, and reference is made to an article titled "Low Temperature Selective Epitaxial Growth of Silicon at Atmospheric Pressure", by T. O. Sedgwick et al., Applied Physics Letters, Vol. 54, No. 26, pages 2689-2691 (1989). Such processes are limited to use of high temperatures, chlorinated silanes, and certain substrates. The high temperature requirement limits the application of such processes, whereas a low temperature process can be applied, for instance, to amorphous silicon technology or to limit dopant diffusion in silicon devices with small dimensions.
It is also known in the plasma-enhanced chemical vapor deposition process to rotate a substrate within a multiplasma-zone apparatus into exposure with spatially-separated zones of hydrogen plasma and hydrogen:silane plasma surrounded by an outer chamber containing no plasma, so as to expose the substrate to silane plasma, to deposit silicon on the substrate, and to expose the silicon deposit to hydrogen plasma, to passivate or modify the silicon surface, the plasma exposures being separated by rotation of the substrate out of exposure to either plasma. The effect of etching on the growth rate of the deposit is negligible, and no selective deposition is disclosed. Reference is made to an article titled "Preparation of highly photoconductive hydrogenated amorphous silicon carbide films with a multiplasma-zone apparatus" by A. Asano et al. in Journal of Applied Physics, Vol. 65, No. 6, 15 Mar. 1989, pages 2439-2944, and to an article titled "Effects of Hydrogen Atoms on the Network Structure of Hydrogenated Amorphous and Microcrystalline Silicon Thin Films" by A. Asano in Applied Physical Letters, Vol. 56, No. 6, pages 533 to 535, Feb. 5, 1990.
The processes disclosed by Asano have one or more disadvantages which are avoided by the present invention. The rotating-substrate apparatus used by Asano is complicated, appears to introduce contaminants such as oxygen, and is slow in operation. More importantly, the Asano process removes the substrate out of the plasma zones after each deposition and surface modification exposure, thereby interrupting the surface reactions produced by exposure to hydrogen plasma, such as etching, hydrogen radical assistance of the CVD deposition film growth, long range relocation for crystallization, formation of microcrystalline silica, etc.
It is also known to selectively deposit microcrystalline silicon:hydrogen:fluorine on crystalline and polysilicon surfaces, while not depositing on thermal silica sidewalls, to produce a self-aligned TFT structure in a plasma-enhanced CVD process, and reference is made to an article titled "Selective Deposition Of N+ Doped MC-Si:H:F By RF Plasma CVD On Si And SiO.sub.2 Substrates", by K. Baert et al., in Materials Research Society Proceedings, Vol. 164, pp. 359-364 (1990). Such process balances etching from SiF.sub.4, versus deposition from SiH.sub.4 at temperatures above 300.degree. C., namely 315.degree.-330.degree. C. SiF.sub.4 causes substantial etching of the SiO.sub.2 and therefore is undesirable. Also it is desirable to be able to produce selective deposition at temperatures below about 300.degree. C., preferably in the area of 250.degree. C., to apply the technology to amorphous silicon substrates.
Prior known processes for depositing silicon on a substrate have one or more deficiencies which render them unsatisfactory or inoperative for the selective deposition of silicon on predetermined areas of a substrate, at low temperatures of 300.degree. C. or less in a conventional PECVD system using only hydrogen and silane gas plasma without requiring the use of fluorinated gas species.